"By September the book was going well – 30,000 words done. A Christmas deadline loomed. I was writing a chapter on the NSA's close, and largely hidden, relationship with Silicon Valley. I wrote that Snowden's revelations had damaged US tech companies and their bottom line. Something odd happened. The paragraph I had just written began to self-delete. The cursor moved rapidly from the left, gobbling text. I watched my words vanish. When I tried to close my OpenOffice file the keyboard began flashing and bleeping.

"Over the next few weeks these incidents of remote deletion happened several times. There was no fixed pattern but it tended to occur when I wrote disparagingly of the NSA. All authors expect criticism. But criticismbefore publication by an anonymous, divine third party is something novel. I began to leave notes for my secret reader. I tried to be polite, but irritation crept in. Once I wrote: "Good morning. I don't mind you reading my manuscript – you're doing so already – but I'd be grateful if you don't delete it. Thank you." There was no reply."